Chlorophyll high-temperature thermoluminescence emission as an indicator of oxidative stress: perturbating effects of oxygen and leaf water content.

A high-temperature chlorophyll thermoluminescence emission can be observed in plant leaves, without preillumination, following various oxidative stresses. This emission was recorded from 0 degrees C to 160 degrees C on a leaf sample pressed by a teflon ring on a heating plate. A band at 140 degrees C was observed in senescing tree leaves, a band at 130 degrees C with a shoulder at 75 degrees C in tobacco leaves treated with the fungal elicitor cryptogein. Measurements in pure O2 or N2 atmospheres did not affect the 130/140 degrees C band, although oxygen increased the thermoluminescence emission at higher temperatures and should be consequently eliminated during measurements. Preventing desiccation by covering the sample with a glass window suppressed the 130/140 degrees C band, except for its rising edge at about 80/90 degrees C. This broad 130/140 degrees C band results from the chlorophyll-exciting thermolysis of lipid peroxides, but it disappears when water is kept in the sample up to 100 degrees C, due to a non-radiative hydrolysis of the peroxides. Therefore, high-temperature thermoluminescence measurements should be performed on leaf samples allowed to dry under an oxygen-free atmosphere.